Network Working Group L. Howard
Internet-Draft Retevia
Intended status: Informational April 25, 2018
Expires: October 27, 2018
Reverse DNS in IPv6 for Internet Service Providersdraft-ietf-dnsop-isp-ip6rdns-05
Abstract
In IPv4, Internet Service Providers (ISPs) commonly provide IN-
ADDR.ARPA information for their customers by prepopulating the zone
with one PTR record for every available address. This practice does
not scale in IPv6. This document analyzes different approaches and
considerations for ISPs in managing the ip6.arpa zone for IPv6
address space assigned to many customers.
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This Internet-Draft will expire on October 27, 2018.
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Copyright (c) 2018 IETF Trust and the persons identified as the
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residential users, maintenance of individual PTR records is
impractical. Administrators at ISPs should consider whether customer
PTR records are needed, and if so, evaluate methods for responding to
reverse DNS queries in IPv6.
1.1. Reverse DNS in IPv4
ISPs that provide access to many residential users typically assign
one or a few IPv4 addresses to each of those users, and populate an
IN-ADDR.ARPA zone with one PTR record for every IPv4 address. Some
ISPs also configure forward zones with matching A records, so that
lookups match. For instance, if an ISP Example.com aggregated
192.0.2.0/24 at a network hub in Town in the province of AnyWhere,
the reverse zone might look like:
1.2.0.192.IN-ADDR.ARPA. IN PTR 1.string.region.example.com.
2.2.0.192.IN-ADDR.ARPA. IN PTR 2.string.region.example.com.
3.2.0.192.IN-ADDR.ARPA. IN PTR 3.string.region.example.com.
.
.
.
254.2.0.192.IN-ADDR.ARPA. IN PTR 254.string.region.example.com.
The conscientious Example.com might then also have a zone:
1.string.region.example.com. IN A 192.0.2.1
2.string.region.example.com. IN A 192.0.2.2
3.string.region.example.com. IN A 192.0.2.3
.
.
.
254.string.region.example.com. IN A 192.0.2.254
Many ISPs generate PTR records for all IP addresses used for
customers, and many create the matching A record.
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Internet-Draft draft-ietf-dnsop-isp-ip6rdns April 20181.2. Reverse DNS Considerations in IPv6
A sample entry for 2001:0db8:0f00:0000:0012:34ff:fe56:789a might be:
a.9.8.7.6.5.e.f.f.f.4.3.2.1.0.0.0.0.0.0.0.0.f.0.8.b.d.0.1.0.0.2
.IP6.ARPA. IN PTR 1.string.region.example.com.
ISPs will often delegate an IPv6 prefix to their customers. Since
2^^80 possible addresses could be configured in an example
2001:db8:f00/48 zone alone, even with automation it is impractical to
write a zone with every possible address entered. If 1000 entries
could be written per second, the zone would still not be complete
after 38 trillion years.
Furthermore, it is often impossible to associate host names and
addresses, since the 64 bits in the Interface Identifier portion of
the address are frequently assigned using SLAAC [RFC4862] when the
host comes online, and may be short-lived.
[RFC1912] is an informational document that says "PTR records must
point back to a valid A record" and further that the administrator
should "Make sure your PTR and A records match." [RFC1912] This
document considers how to follow this advice for AAAA and PTR
records.
2. Alternatives in IPv6
Several options exist for providing reverse DNS in IPv6. All of
these options also exist for IPv4, but the scaling problem is much
less severe in IPv4. Each option should be evaluated for its scaling
ability, its compliance with existing standards and best practices,
and its availability in common systems.
2.1. Negative Response
Some ISP DNS administrators may choose to provide only a NXDomain
response to PTR queries for subscriber addresses. In some ways, this
is the most accurate response, since no name information is known
about the host. Providing a negative response in response to PTR
queries does not satisfy the expectation in [RFC1912] for entries to
match. Users of services which are dependent on a successful lookup
will have a poor experience. For instance, some web services and SSH
connections wait for a DNS response, even NXDOMAIN, before
responding. For best user experience, then, it is important to
return a response, rather than have a lame delegation. On the other
hand, external mail servers are likely to reject connections, which
might be an advantage in fighting spam. DNS administrators should
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consider the uses for reverse DNS records and the number of services
affecting the number of users when evaluating this option.
2.2. Wildcard match
The use of wildcards in the DNS is described in [RFC4592], and their
use in IPv6 reverse DNS is described in [RFC4472].
While recording all possible addresses is not scalable, it may be
possible to record a wildcard entry for each prefix assigned to a
customer. Consider also that "inclusion of wildcard NS RRSets in a
zone is discouraged, but not barred." [RFC4035]
This solution generally scales well. However, since the response
will match any address in the wildcard range (/48, /56, /64, etc.), a
forward DNS lookup on that response given will not be able to return
the same hostname. This method therefore fails the expectation in
[RFC1912] for forward and reverse to match. DNSsec [RFC4035]
scalability is limited to signing the wildcard zone, which may be
satisfactory.
2.3. Dynamic DNS
One way to ensure forward and reverse records match is for hosts to
update DNS servers dynamically, once interface configuration (whether
SLAAC, DHCPv6, or other means) is complete, as described in
[RFC4472]. Hosts would need to provide both AAAA and PTR updates,
and would need to know which servers would accept the information.
This option should scale as well or as poorly as IPv4 dynamic DNS
does. Dynamic DNS may not scale effectively in large ISP networks
which have no single master name server, but a single master server
is not best practice. The ISP's DNS system may provide a point for
Denial of Service attacks, including many attempted dDNS updates.
Accepting updates only from authenticated sources may mitigate this
risk, but only if authentication itself does not require excessive
overhead. No authentication of dynamic DNS updates is inherently
provided; implementers should consider use of TSIG [RFC2845], or at
least ingress filtering so updates are only accepted from customer
address space from internal network interfaces, rate limit the number
of updates from a customer per second, and consider impacts on
scalability. UDP is allowed per [RFC2136] so transmission control is
not assured, though the host should expect an ERROR or NOERROR
message from the server [RFC2136]; TCP provides transmission control,
but the updating host would need to be configured to use TCP.
Administrators should consider what domain will contain the records,
and who will provide the names. If subscribers provide hostnames,
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they may provide inappropriate strings. Consider "ihate.example.com"
or "badword.customer.example.com" or
"celebrityname.committed.illegal.acts.example.com."
There is no assurance of uniqueness if multiple hosts try to update
with the same name ("mycomputer.familyname.org"). There is no
standard way to indicate to a host what server it should send dDNS
updates to; the master listed in the SOA is often assumed to be a
dDNS server, but this may not scale.
2.3.1. Dynamic DNS from Individual Hosts
In the simplest case, a residential user will have a single host
connected to the ISP. Since the typical residential user cannot
configure IPv6 addresses and resolving name servers on their hosts,
the ISP should provide address information conventionally (i.e.,
their normal combination of RAs, DHCP, etc.), and should provide a
DNS Recursive Name Server and Domain Search List as described in
[RFC3646] or [RFC6106]. In determining its Fully Qualified Domain
Name, a host will typically use a domain from the Domain Search List.
This is an overloading of the parameter; multiple domains could be
listed, since hosts may need to search for unqualified names in
multiple domains, without necessarily being a member of those
domains. Administrators should consider whether the domain search
list actually provides an appropriate DNS suffix(es) when considering
use of this option. For purposes of dynamic DNS, the host would
concatenate its local hostname (e.g., "hostname") plus the domain(s)
in the Domain Search List (e.g., "customer.example.com"), as in
"hostname.customer.example.com."
Once it learns its address, and has a resolving name server, the host
must perform an SOA lookup on the ip6.arpa record to be added, to
find the owner, eventually to find the server authoritative for the
zone (which might accept dynamic updates). Several recursive lookups
may be required to find the longest prefix which has been delegated.
The DNS administrator must designate the Primary Master Server for
the longest match required. Once found, the host sends dynamic AAAA
and PTR updates using the concatenation defined above
("hostname.customer.example.com").
In order to use this alternative, hosts must be configured to use
dynamic DNS. This is not default behavior for many hosts, which is
an inhibitor for the large ISP. This option may be scalable,
although registration following an outage may cause significant load,
and hosts using privacy extensions [RFC4941] may update records
daily. It is up to the host to provide matching forward and reverse
records, and to update them when the address changes.
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Internet-Draft draft-ietf-dnsop-isp-ip6rdns April 20182.3.2. Dynamic DNS through Residential Gateways
Residential customers may have a gateway, which may provide DHCPv6
service to hosts from a delegated prefix. ISPs should provide a DNS
Recursive Name Server and Domain Search List to the gateway, as
described above and in [RFC3646] and [RFC6106]. There are two
options for how the gateway uses this information. The first option
is for the gateway to respond to DHCPv6 requests with the same DNS
Recursive Name Server and Domain Search List provided by the ISP.
The alternate option is for the gateway to relay dynamic DNS updates
from hosts to the servers and domain provided by the ISP. Host
behavior is unchanged; the host sends the same dynamic updates,
either to the ISP's server (as provided by the gateway), or to the
gateway for it to forward.
2.3.3. Automatic DNS Delegations
An ISP may delegate authority for a subdomain such as
"customer12345.town.AW.customer.example.com" or
"customer12345.example.com" to the customer's gateway. Each domain
thus delegated must be unique within the DNS. The ISP may also then
delegate the ip6.arpa zone for the prefix delegated to the customer,
as in (for 2001:db8:f00::/48) "0.0.f.0.8.b.d.0.1.0.0.2.ip6.arpa."
Then the customer could provide updates to their own gateway, with
forward and reverse. However, individual hosts connected directly to
the ISP rarely have the capability to run DNS for themselves;
therefore, an ISP can only delegate to customers with gateways
capable of being authoritative name servers. If a device requests a
DHCPv6 Prefix Delegation, that may be considered a reasonably
reliable indicator that it is a gateway, rather than an individual
host. It is not necessarily an indicator that the gateway is capable
of providing DNS services, and therefore cannot be relied upon as a
way to test whether this option is feasible. In fact, this kind of
delegation will not work for devices complying with [RFC6092], which
includes the requirement, "By DEFAULT, inbound DNS queries received
on exterior interfaces MUST NOT be processed by any integrated DNS
resolving server."
If the customer's gateway is the name server, it provides its own
information to hosts on the network, as often done for enterprise
networks, and as described in [RFC2136].
An ISP could provide authoritative responses as a secondary server to
the customer's master server. For instance, the home gateway name
server could be the master server, with the ISP providing the only
published NS authoritative servers.
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To implement this alternative, users' residential gateways must be
capable of acting as authoritative name servers capable of dynamic
DNS updates. There is no mechanism for an ISP to dynamically
communicate to a user's equipment that a zone has been delegated, so
user action would be required. Most users have neither the equipment
nor the expertise to run DNS servers, so this option is unavailable
to the residential ISP.
2.3.4. Generate Dynamic Records
An ISP's name server that receives a dynamic forward or reverse DNS
update may create a matching entry. Since a host capable of updating
one is generally capable of updating the other, this should not be
required, but redundant record creation will ensure a record exists.
ISPs implementing this method should check whether a record already
exists before accepting or creating updates.
This method is also dependent on hosts being capable of providing
dynamic DNS updates, which is not default behavior for many hosts.
2.3.5. Populate from DHCP Server
A ISP's DHCPv6 server may populate the forward and reverse zones when
the DHCP request is received, if the request contains enough
information. [RFC4704]
However, this method will only work for a single host address
(IA_NA); the ISP's DHCP server would not have enough information to
update all records for a prefix delegation. If the zone authority is
delegated to a home gateway which used this method, the gateway could
update records for residential hosts. To implement this alternative,
users' residential gateways would have to support the FQDN DHCP
option, and would have to either have the zones configured, or send
dDNS messages to the ISP's name server.
2.3.6. Populate from RADIUS Server
A user may receive an address or prefix from a RADIUS [RFC2865]
server, the details of which may be recorded via RADIUS Accounting
[RFC2866] data. The ISP may populate the forward and reverse zones
from the accounting data if it contains enough information. This
solution allows the ISP to populate data concerning allocated
prefixes (as per 2.2 (wildcards)) and CPE endpoints, but as with
2.3.5 does not allow the ISP to populate information concerning
individual hosts.
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Internet-Draft draft-ietf-dnsop-isp-ip6rdns April 20182.4. Delegate DNS
For customers who are able to run their own DNS servers, such as
commercial customers, often the best option is to delegate the
reverse DNS zone to them, as described in [RFC2317] (for IPv4).
However, since most residential users have neither the equipment nor
the expertise to run DNS servers, this method is unavailable to
residential ISPs.
This is a general case of the specific case described in
Section 2.3.3. All of the same considerations still apply.
2.5. Dynamically Generate PTR When Queried ('On the Fly')
Common practice in IPv4 is to provide PTR records for all addresses,
regardless of whether a host is actually using the address. In IPv6,
ISPs may generate PTR records for all IPv6 addresses as the records
are requested. Several DNS servers are capable of this.
An ISP using this option should generate a PTR record on demand, and
cache or prepopulate the forward (AAAA) entry for the duration of the
time-to-live of the PTR. Similarly, the ISP would prepopulate the
PTR following a AAAA query. Alternatively, if an algorithm is used
to generate unique name, it can be employed on the fly in both
directions. This option has the advantage of assuring matching
forward and reverse entries, while being simpler than dynamic DNS.
Administrators should consider whether the lack of user-specified
hostnames is a drawback. It may be possible to allow user-specified
hostnames for some records and generate others on the fly; looking up
a record before generating on the fly may slow responses or may not
scale well.
DNSsec [RFC4035] signing records on the fly may increase load;
unsigned records can indicate that these records are less trusted,
which might be acceptable.
Another consideration is that the algorithm used for generating the
record must be the same on all servers for a zone. In other words,
any server for the zone must produce the same response for a given
query. Administrators managing a variety of rules within a zone
might find it difficult to keep those rules synchronized on all
servers.
3. Manual User Updates
It is possible to create a user interface, such as a web page, that
would allow end users to enter a hostname to associate with an
address. Such an interface would need to be authenticated (only the
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authorized user could add/change/delete entries). It would need to
specify only the host bits if the ISP changes prefixes assigned to
customers, and the back end would therefore need to be integrated
with prefix assignments, so that when a new prefix was assigned to a
customer, the DNS service would look up user-generated hostnames and
delete the old record and create the new one.
Considerations about some records being static and others dynamic or
dynamically generated (section 2.5) and the creativity of users
(section 2.3) still apply.
4. Considerations and Recommendations
There are six common uses for PTR lookups:
Rejecting mail: A PTR with a certain string or missing may indicate
"This host is not a mail server," which may be useful for rejecting
probable spam. The absence of a PTR leads to the desired behavior.
Serving ads: "This host is probably in town.province." An ISP that
does not provide PTR records might affect somebody else's
geolocation.
Accepting SSH connections: The presence of a PTR may be inferred to
mean "This host has an administrator with enough clue to set up
forward and reverse DNS." This is a poor inference.
Log files: Many systems will record the PTR of remote hosts in their
log files, to make it easier to see what network the remote host uses
when reading logs later.
Traceroute: The ability to identify an interface and name of any
intermediate node or router is important for troubleshooting.
Service discovery: [RFC6763] specifies "DNS-based Service Discovery"
and section 11 specifically describes how PTRs are used.
As a general guideline, when address assignment and name are under
the same authority, or when a host has a static address and name,
AAAA and PTR records should exist and match. For residential users,
if these four use cases are important to the ISP, the administrator
will then need to consider how to provide PTR records.
The best accuracy would be achieved if ISPs delegate authority along
with address delegation, but residential users rarely have domain
names or authoritative name servers.
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Dynamic DNS updates can provide accurate data, but there is no
standard way to indicate to residential devices where to send
updates, if the hosts support it, and if it scales.
An ISP has no knowledge of its residential users' hostnames, and
therefore can either provide a wildcard response or a dynamically
generated response. A valid negative response (such as NXDomain) is
a valid response, if the four cases above are not essential; lame
delegation should be avoided.
5. Security and Privacy Considerations5.1. Using Reverse DNS for Security
Some people think the existence of reverse DNS records, or matching
forward and reverse DNS records, provides useful information about
the hosts with those records. For example, one might infer that the
administrator of a network with properly configured DNS records was
better-informed, and by further inference more responsible, than the
administrator of a less-thoroughly configured network. For instance,
most email providers will not accept incoming connections on port 25
unless forward and reverse DNS entries match. If they match, but
information higher in the stack (for instance, mail source) is
inconsistent, the packet is questionable. These records may be
easily forged though, unless DNSsec or other measures are taken. The
string of inferences is questionable, and may become unneeded if
other means for evaluating trustworthiness (such as positive
reputations) become predominant in IPv6.
Providing location information in PTR records is useful for
troubleshooting, law enforcement, and geolocation services, but for
the same reasons can be considered sensitive information.
5.2. DNS Security with Dynamic DNS
Security considerations of using dynamic DNS are described in
[RFC3007]. DNS Security Extensions are documented in [RFC4033].
Interactions with DNSsec are described throughout this document.
5.3. Considerations for Other Uses of the DNS
Several methods exist for providing encryption keys in the DNS. Any
of the options presented here may interfere with these key
techniques.
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